Abu Bakr As-Siddiq said to the Prophet, "Teach me an invocation with which I may invoke (Allah) in my prayer." The Prophet ﷺ said, "Say: Allahumma inni zalamtu nafsi zulman kathiran wala yaghfirudhdhunuba illa anta, Faghfirli maghfiratan min indika war-hamni, innaka antalGhafur-Rahim."  

عَنْ أَبِي بَكْرٍ الصِّدِّيقِ ؓ أَنَّهُ قَالَ لِلنَّبِيِّ ﷺ عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلاَتِي قَالَ قُلِ اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلاَ يَغْفِرُ الذُّنُوبَ إِلاَّ أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ وَقَالَ عَمْرٌو عَنْ يَزِيدَ عَنْ أَبِي الْخَيْرِ إِنَّهُ سَمِعَ عَبْدَ اللَّهِ بْنَ عَمْرٍو قَالَ أَبُو بَكْرٍ ؓ لِلنَّبِيِّ ﷺ  

The Verse: 'Neither say your prayer aloud, nor say it in a low tone.' (17.110) was revealed as regards invocation.  

عَنْ عَائِشَةَ {وَلاَ تَجْهَرْ بِصَلاَتِكَ وَلاَ تُخَافِتْ بِهَا} أُنْزِلَتْ فِي الدُّعَاءِ  

We used to say in the prayer: 'AsSalam be on Allah, As-Salam be on so-and so.' So one day the Prophet said to us, "Allah Himself is As-Salam; when anyone of you sits during his prayer, he should say: 'at-tah, iyyatu-li l-lahi,' up to 'As-Salihin,' (All the compliments are for Allah ...righteous people) for when he recites this, then he says his Salam to all the righteous people present in the heavens and on the earth. Then he should say, 'I testify that none has the right to be worshipped except Allah, and that Muhammad is His slave and His Apostle,' and then he can select whatever he likes to celebrate (Allah's) Praises."  

كُنَّا نَقُولُ فِي الصَّلاَةِ السَّلاَمُ عَلَى اللَّهِ السَّلاَمُ عَلَى فُلاَنٍ فَقَالَ لَنَا النَّبِيُّ ﷺ ذَاتَ يَوْمٍ إِنَّ اللَّهَ هُوَ السَّلاَمُ فَإِذَا قَعَدَ أَحَدُكُمْ فِي الصَّلاَةِ فَلْيَقُلِ التَّحِيَّاتُ لِلَّهِ إِلَى قَوْلِهِ الصَّالِحِينَ فَإِذَا قَالَهَا أَصَابَ كُلَّ عَبْدٍ لِلَّهِ فِي السَّمَاءِ وَالأَرْضِ صَالِحٍ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ ثُمَّ يَتَخَيَّرُ مِنَ الثَّنَاءِ مَا شَاءَ